Bump to using SQLAlchemy migrate 0.8.2.

The motivation here is to be able to remove
a custom check for 0.7.3 in nova. Bumping
this requirement will let us do that.

NOTE: version 0.8.1 had a borked version number so
we require 0.8.2 or greater due to that.

Change-Id: Icde6248ca102884323d7912bab0f4a716aa294ec
Merge "Bump to using SQLAlchemy migrate 0.8.2."